The disclosure of Japanese Patent Application No. 2001-226728 filed on Jul. 26, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to control system and method for a vehicle including a continuously variable transmission coupled to an output side of an internal combustion engine, such as a gasoline engine or a diesel engine, and more particularly to system and method for controlling the output power of the internal combustion engine when a request for an increase of the power is made.
2. Description of Related Art
In recent years, continuously variable transmissions have been widely used as transmissions of vehicles. The continuously variable transmission is capable of changing the speed ratio continuously, and therefore is able to precisely control the engine speed as an input rotation speed of the transmission. Also in recent years, the throttle opening, fuel injection quantity, and supercharging can be electrically controlled. Thus, by using the continuously variable transmission to control the engine speed while electrically controlling the engine load, it is possible to operate the an internal combustion engine in a desirable operating state (an operating point) which provides the best fuel efficiency or minimized fuel consumption.
One example of a control system for an internal combustion engine of the above type coupled to a continuously variable transmission is disclosed in Japanese Laid-Opened Patent Publication No. 2000-289496. In the control system disclosed in this publication, a target driving force is first determined on the basis of a required output power as represented by an accelerator pedal position (i.e., an amount of depression of an accelerator pedal) and the vehicle speed detected at the time of the control (which will be referred to as xe2x80x9ccurrent vehicle speedxe2x80x9d). A target output power is then determined on the basis of the determined target driving force and the current vehicle speed. In accordance with the determined target output power, on one hand, a target engine speed that will provide the best fuel efficiency is determined based on a map prepared in advance, and the speed ratio of the continuously variable transmission is controlled so as to achieve the target engine speed. On the other hand, a target output torque is determined in accordance with the target output power and the target engine speed, and the engine load is controlled so as to achieve the target output torque.
According to the above-described control, the internal combustion engine is maintained in an operating state which provides the best fuel efficiency. With this control in which an importance is given to the fuel efficiency, the driving force is not immediately increased when the accelerator pedal is pressed down by a large degree, resulting in deterioration of an acceleration response. In the control system as disclosed in the above publication, therefore, when a large output power is required, the engine torque and engine speed are controlled in the following manner, for example. Namely, the engine torque is first increased to the maximum level (WOT). After the target output power is reached, the engine torque and the engine speed are controlled so as to bring the operating state of the engine to a final target operating point (which is determined based on the required output power) that lies on an optimum fuel efficiency curve, while maintaining the achieved output power.
FIG. 5 shows changes or movements of the operating point of the engine during the control as described above. FIG. 5 indicates equi-fuel-efficiency lines, an equi-power line on which the output power of the engine is equal, and an optimum fuel efficiency curve, by using the engine speed and torque as parameters. In FIG. 5, xe2x80x9cP1xe2x80x9d represents an operating state of the internal combustion engine in which the engine speed is at the lower limit value, for example, when the engine is in an idling state. When a request for increased output power is made at P1, for example, when an accelerator pedal (not shown) is depressed by a large degree while the engine is at the operating point P1, an operating point P4 on the optimum fuel efficiency curve is determined as the final target operating point on the basis of the required output power. As described above, an acceleration response may deteriorate if the operating state of the engine is controlled so as to change along the optimum fuel efficiency curve. Upon receipt of a request for acceleration, therefore, the engine torque is initially controlled to the upper limit value so that the operating point of the engine reaches P2 as shown in FIG. 5.
While the engine torque cannot exceed the upper limit value, the vehicle speed gradually increases and shifting (downshift) occurs due to the increased engine torque, whereby the engine speed gradually increases. In this process, the operating point shifts along a curve representing the upper limit values of the engine torque. When the operating state of the engine has reached a point (denoted by P3 in FIG. 5) at which the curve representing the upper limit values of the engine torque and the equi-power line passing through the final target operating point intersect, the engine speed and the engine torque are changed toward the final target operating point P4 along the equi-power line.
When a large output power is demanded, the difference between the current torque at the operating point P1 and the upper limit torque at the operating point P2 is large and the difference between the engine speed at the operating point P1 and that at the final target operating point P4 is also large. When the difference in the engine torque is large, it takes some time to change the engine torque from the operating point P1 to the operating point P2 due to a delay in an increase of the engine torque which unavoidably arises for a mechanical reason, or the like.
The above-described situation may also occur with an internal combustion engine including a turbocharger. More specifically, as shown in FIG. 6, the boost pressure of the turbocharger starts increasing immediately after the accelerator pedal is depressed (at point t1) and continues to increase gradually until it reaches the target boost pressure at point t2 after a certain period of time. This time period is a delay time generally called xe2x80x9cturbo lag.xe2x80x9d In the internal combustion engine including the turbocharger, therefore, it takes some time for the engine torque to reach the upper limit value due to the turbo lag.
In the meantime, the rate of change of the speed ratio of the continuous variable transmission has the upper limit, as schematically shown in FIG. 7. More specifically, the rate of increase of the speed ratio, or the shift speed, is limited by mechanical or structural conditions of, for example, a hydraulic system. Also, since the rotation speed of certain rotating members change with the speed ratio, the rate of change of the speed ratio is limited by the inertial force which arises upon a change of the rotation speed. With the shift speed thus limited, it takes some time to change the engine speed from the operating point P2 to the operating point P3 or to the final target operating point P4.
In sum, when the vehicle is to be accelerated, the known system initially performs control for increasing the engine torque with a response delay, while keeping a constant engine speed. Subsequently, the system performs shift control (i.e., control of the speed ratio of the CVT) with a response delay, while keeping the engine torque at the upper limit value. Finally, the engine torque and engine speed are controlled along the equi-power line passing the final target operating point. Thus, the delays in the controls for increasing the engine toque and increasing the engine speed amount to a total delay that occurs at the time of the acceleration. Due to the acceleration delay, an acceleration response has not been sufficiently improved, though there is room for further improvement.
It is one object of the invention to provide a control system for a vehicle, which is capable of achieving a desired output power appropriately in response to a request for an increase in the output power.
To accomplish the above and/or other object(s), there is provided according to one aspect of the invention a control system for a vehicle in which an internal combustion engine whose output torque is controlled based on a required output power is coupled to a continuously variable transmission whose input rotation speed is controlled based on the required output power. A controller of the control system determines a final target operating point of the internal combustion engine which is defined by the output torque and the engine speed, on the basis of the required output power, sets a transient operating point of the engine to one of possible operating points that can be achieved within a predetermined period of time such that the operating point of the engine approaches the final target operating point, and controls the output torque and the input rotation speed of the continuously variable transmission so as to operate the internal combustion engine at the set transient operating point.
In the control system constructed as described above, when there is a demand for an increase of the output power of the engine, the final target operating point, which is defined by the output torque and the engine speed, is determined based on the required output power. If the final target operating point cannot be reached within a predetermined period of time, the engine is controlled to a transient operating point. The transient operating point is a predetermined operating point which is selected from possible operating points that can be reached within the predetermined period of time, and which is closer to the final target operating point than the current operating point. Then, the output torque of the engine and the engine speed to be established by the CVT are controlled so as to coincide with the transient operating point. Thus, both of the engine torque and the engine speed are changed at the same time toward the final target operating point, and therefore the operating state of the engine can be changed so as to achieve the required output power with high accuracy, even in the presence of a delay in a change of at least one of the engine torque and the engine speed.